Fastener driving tool

ABSTRACT

A fastener driving tool including a piston having a driving element supported eccentric to the axis thereof and reciprocated in a bore by fluid under pressure. A contactor mechanism comprises an inner portion located in the bore in the path of movement of the piston and an outer portion defining a fastener outlet aligned with the path of movement of the driving element. Where the contactor mechanism is a dimpler mechanism, the outer portion has a spherical dimpling surface surrounding the outlet and an expansive energy dissipating surface surrounding the dimpler surface. The dimpler mechanism and piston have cooperating means that cause a deceleration of the piston and an acceleration of the dimpler, rigid striking engagement between the piston and dimpler mechanism being made when their speed or velocity differential has been substantially reduced. The tool also incorporates a safety mechanism that prevents firing until the tool is in engagement with a workpiece, and includes shock absorbing means to prevent damage to the tool when inadequate back-up exists.

United States Patent [1 1 Golsch FASTENER DRIVING TOOL [75] Inventor:Rudolf A. M. Golsch, Des Plaines,

Ill.

[73] Assignee: Signode Corporation, Glenview, Ill.

[22] Filed: Sept. 13, 1971 [21] Appl. No.: 179,902

Primary ExaminerGranville Y. Custer, Jr. Att0meyMacDressler et al.

[451 Nov. 27, 1973 [57 ABSTRACT A fastener driving tool including apiston having a driving element supported eccentric to the axis thereofand reciprocated in a bore by fluid under pressure. A contactormechanism comprises an inner portion located in the bore in the path ofmovement of the piston and an outer portion defining a fastener outletalinged with the path of movement of the driving element. Where thecontactor mechanism is a dimpler mechanism, the outer portion has aspherical dimpling surface surrounding the outlet and an expansiveenergy dissipating surface surrounding the dimpler surface. The dimplermechanism and piston have 00- operating means that cause a decelerationof the piston and an acceleration of the dimpler, rigid strikingengagement between the piston and dimpler mechanism being made whentheir speed or velocity differential has been substantially reduced. Thetool also incorporates a safety mechanism that prevents firing until thetool is in engagement with a workpiece, and includes shock absorbingmeans to prevent damage to the tool when inadequate back-up exists.

30 Claims, 10 Drawing Figures Patnted Nov. 27, 1973 3 Sheets-Sheet 1Patentd Nov. 27, 1973 5 Sheets-Sheet om A6 Patented Nov. 27, 1973 3Sheets-Sheet 3 1 FASTENER DRIVING TOOL BACKGROUND OF THE INVENTION Thepresent invention relates generally to pneumatically actuated fastenerdrivers and more particularly to an improved fastener driver tool thatdrives a fastener and simultaneously forms a recess or dimple around thefastener.

Pneumatically operated fastener driving tools have been used in asubstantial number of industries. Tools of this type have been used todrive fasteners such as staples, tacks, T-nails, and ordinary headednails through readily pierceable materials.

One application for such tools that has great commercial potential is indriving fasteners for securing wallboard or dry wall, as to woodensupporting structures, such as studs and rafters. Since the use of drywall and wallboard has become very extensive, especially in theprefabricated building industry recently, the need for such a practicaltool has become very acute.

In securing wallboard to a wooden structure, it is highly desirable thatthe heads of the fasteners be countersunk and that a limited area of thesurface of the wallboard, the area about the countersunk fastener head,he formed with a uniform but slight depression to i receive a hardenablefilter material so that the head of the fastener is not visible in thefinished structure. The countersinking must be accomplished withouttearing or ripping the surface, such as the paper surface, of thewallboard.

In order to accomplish this, several fastening tools have been proposed,each of which incorporates a dimpling head or dimpler mechanism that isactuated by the fastener driving mechanism to provide a shallow recessor dimple in the wallboard surface concurrently with the driving of afastener into the wallboard. Examples of such tools are disclosed in thefollowing US. Pat: Bacon et al. No. 2,679,044; Smith 2,918,675; Nelson3,027,560; and Michel 3,040,327.

The tools disclosed in these patents utilize two basic types of drivingmeans for the dimpler mechanism. The first type, which may be referredto as the direct impact type, has the driving piston, the carrier of thekinetic energy, strike the dimpler in a direct metal-to-metal impact.The second type of drive mechanism utilizes a resilient member carriedby the driving piston to soften the impact of the piston against thedimpler.

The advantage of the direct impact type is that the extent ofpenetration of the fastener relative to the dimpler can be accuratelycontrolled. However, there are a number of disadvantages in a directimpact drive arrangement that overshadow the principal advantage.Because this arrangement requires metal-to-metal contact between thepiston driver and the dimpler, the impact speed of the driver has to beconsiderably lower than in customary pneumatic fastener driving tools toreduce the shock and extreme stresses and to provide a reasonableservice life for the various parts. This means that there is either morerecoil of the tool or more weight must be added to the housing portion.If more weight is added to reduce recoil, the tool becomes heavy andawkward to handle. Of even greater importance is the fact that when nofastener is located in the tool and the tool is actuated, the shockresulting from the direct impact between the drive piston and thedimpler fatigues the parts and can readily result in destruction ofeither or both of the parts.

The second type of drive mechanism discussed above, i.e., that utilizinga resilient member, while reducing the shock loads and softening theimpact between the driver and the dimpler by eliminating directmetal-to-metal contact, minimizes the degree of control over theterminal movement of the dimpler mechanism relative to the fastener andfastener driver and therefore control over the depth of the dimple anddepth of penetration of the fastener.

A further disadvantage of all of the driving tools that have beenproposed for driving a fastenerand for simultaneously dimpling aworkpiece surface, is that the tools were not capable of compensatingfor variations in the driving resistance for the fastener which mayresult from variations in fluid pressure, and from variations in suchcharacteristics as grain structure, moisture content, knots, etc., inthe lumber and studs to which the workpiece (such as a wallboard) issecured. That is of extreme importance when securing wallboard in areaswhere a clean finished surface is desired. For example, if a nailhappens to strike a hard spot like a knot, its head might protrude fromthe surface and the wallboard would not be held tightly against asupporting stud. On the other hand, if a nail is driven too deeply, thewallboard core may be crushed or the sheathing may even be torn orruptured, conditions which are unacceptable for known reasons. Sincewallboard generally includes gypsum or other material of littlestructural strength covered on both sides with a sheathing of paper, therupturing of the paper sheathing may preclude proper and dependablemounting of the wallboard, and may result in a finished structure thatis unsightly in appearance and that has a limited life, and may fail tomeet building code requirements.

Problems such as these with fastener driving tools previously suggestedfor securing wallboard to studs and the like have been reasonscontributing to the fact that no tools are commercially available tofill a very substantial present need, i.e., a tool which is capable ofdriving a fastener to a predetermined depth while simultaneously forminga recess of predetermined dimension and depth around the fastener head.

7 SUMMARY OF THE INVENTION The present invention overcomes thedisadvantages of the tools that have heretofore been proposed forsimultaneously dimpling a workpiece surface as a fastener is beingdriven.

The fastener tool of the present invention is capable of automaticallycompensating for variations in driving resistance of a fastener. Itminimizes the possibility of damaging the surface of wallboard and makesit possible consistently to push and hold wallboard against a stud orthe like. In addition, the tool is capable of being operated extremelyclose to corners so that virtually all of the fasteners that must bedriven can be driven by the tool, rather than manually. Furthermore, thetool also incorporates mechanism that will prevent free firing, i.e.,firing before the tool is placed against a work surface, and structurethat will minimize the possibility of damage to any of the parts shouldthe tool be fired when no fastener is present or when inadequate backupsupport exists.

The pneumatically operated fastener driving tool includes a driverassembly that is reciprocated within a bore located in a housing and adimpler mechanism supported for reciprocation on the housing. Thedimpler mechanism has an inner portion in the bore located in the pathof movement of a piston portion of the driver assembly. The innerportion of the dimpler mechanism, i.e., the portion closest to thepiston, comprises a metal stud portion extending above a substantiallyflat surface and a resilient member seated on said flat surfacesurrounding the stud portion and extending above the upper surface ofthe stud. The stud portion and resilient member cooperate with thepiston so that the dimpler is initially accelerated via contact betweenthe piston and resilient member while the piston is decelerated, thepiston acting to compress the resilient member. Thereafter, the metalpiston first directly strikes the upper end of the metal stud to provideconcurrent rigid movement of the dimpler mechanism and piston.

In the fastener driving tool of the present invention, a fastenerdriving element is secured to the piston eccentrically of the axis ofthe bore so that the driver element can be located closer to the frontof the housing and as such in closer proximity to corners of workpieces,thereby to drive fasteners closer to corners and the like. In prior arttools, nails at comers had to be driven manually so that they enteredthe wallboard and members to be secured to each other generallyperpendicularly. To counterbalance the weight of the driver element, thedrive piston has counterbalancing means secured to the opposite side.

The tool also advantageously provides means for dissipating excessenergy that is applied to the dimpler after a dimple has been producedin the workpiece, thereby to control the depth of the dimple and itssize. More specifically, the dimpler has a convex or a spherical segmentsurrounding the outlet through which the fastener passes and an annularexpansive collar adjacent to or surrounding the spherical segment.Preferably, the expansive collar is fiat and tapers slightly upwardlyand outwardly away from the spherical segment.

In addition, the tool also incorporates a safety-trigger interlockmechanism which prevents actuation of the tool until the dimplermechanism engages a workpiece. The safety mechanism is actuated inresponse to movement of the dimpler mechanism from its extended positionto its retracted position. In prior art tools it was necessary toretract the dimpler before the trigger could be squeezed, otherwise freefiring of fasteners would take place. With the safety mechanism hereincorporated, the trigger may be squeezed and held and as soon as thetool is placed against a workpiece, it will fire to drive and to dimple,speeding a cycle of operation of the tool.

Further, the tool also incorporates a specially located shock absorbingcushion which minimizes the possibility of damage to any of the partsshould the tool be actuated, for example, when wallboard is not properlylocated with respect to a stud, such as where wallboard is spaced adistance away from a stud to which it is to be fastened, i.e., wheninadequate back-up support exists.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF DRAWINGS FIG. l is a fragmentaryside elevation view, partly in section, of a fastener driving tool ofthis invention;

FIG. 2 is a bottom view taken generally along line 2-2 of FIG. 1;

FIG. 3 is an enlarged fragmentary sectional view showing the tool priorto application against a work surface;

FIG. 4 is a section taken generally along line 44 of FIG. 3;

FIG. 5 is a bottom view of the dimpler mechanism taken generally alongthe line 5-5 of FIG. 3;

FIG. 6 is a view similar to FIG. 3 showing the tool at the end of aworkstroke;

FIG. 7 is a sectional view taken generally along line 77 of FIG. 6;

FIG. 8 is a fragmentary sectional view taken generally along line 88 ofFIG. 6;

FIG. 9 is a sectional view taken generally along line 9-9 of FIG. 6; and

FIG. 10 is a fragmentary rear view of the dimpler mechanism.

DETAILED DESCRIPTION While this invention is susceptible of embodimentin many different forms, there is shown in the drawings and will hereinbe described in detail one specific embodiment, with the understandingthat the present disclosure is to be considered as an exemplification ofthe principles of the invention and is not intended to limit theinvention to the embodiment illustrated.

Referring first to FIGS. 1 and 3, there is illustrated a pneumaticallyoperated fastener driving tool 10 including a housing structure 12having a body portion 14 and a head portion 16 on the forward end of thebody portion. The body portion serves as a handle by which the operatormay grasp and manipulate the tool. The handle portion is cored toprovide a storage chamber 18 into which fluid under pressure, such asair, from a suitable source (not shown) is introduced through an opening20, to provide air to appropriate passages for driving a piston.

The head portion 16, located on the forward end of the housing, has anopening 22 that terminates at a lower end in a nose piece 24. A cylinder26 is secured in the opening 22 and defines a bore 28 in which a driverassembly 30 is reciprocated by the source of fluid under pressure. Thedriver assembly 30 includes driving piston 31 which has a blade ordriver 32 secured thereto.

According to one aspect of the invention, the driver element 32 iseccentrically secured to the piston 31 so that a suitable fastener ornail may be driven into a workpiece in sufficiently close proximity to acorner to meet the requirement of building codes and to minimize thenecessity for hand or manual fastener driving. For example, somebuilding codes require that the corner fasteners must be perpendicularlydriven into wallboard at least 3/8 inch, and not more than 1/2 inch fromthe wallboard edge. In the present tool, this is accomplished byeccentrically or laterally offsetting the driver element with respect tothe central axes of the bore and the piston. As is clearly shown inFIGS. 1, 3 and 6, the driver element 32 is secured to the piston 31between the central axis of the piston and the forward portion of thehousing by an integral stud 34 (FIG. 6) which extends through an opening36 in the piston 31 and which has an enlarged flange 38 which engagesthe lower surface of the piston. The driver element 32 is held in fixedposition on the piston by a nut 40 threaded on the free end of the stud.The forward offset location of the driver element will allow a nail orother fastener to be driven in closer proximity to the forward end ofthe housing, as will be explained in more detail hereinafter.

As will be appreciated, fastening tools of the type to which thisinvention pertains are operated with highpressure fluid and act as gunsthat shoot fasteners rather than bullets. As such, the eccentricmounting and weight of the drive element on the piston will produce aside load on the piston during the driving of a fastener. To minimizethis potential problem and to reduce the wear of the relatively movingparts, piston 31 also mounts a counterweight or counterbalancing means44 between the central axes of the bore and piston and the rear of thehousing structure. As clearly shown in FIGS. 1 and 6, the counterweightor counterbalancing means consists of a sleeve 46 that receives a stud48 which is threaded into an opening 49 in the piston. For example, thesleeve and stud may have a combined weight that is substantially equalto the weight of the driver element 32, and the driver element may beoffset to one side of the central axis of the bore substantially thesame distance that the counterbalancing means is oppositely spaced fromthat axis. In order to further insure that the eccentrically mounteddriver el ement will operate smoothly, the confronting surfaces of thecylinder 26 and the piston 31 are preferably finished with a coating ofhard, smooth, and wear resistant material. With this arrangement, theeccentrically mounted driver element will not significantly adverselyaffect the operation of the tool during the fastener driving operationand will not significantly adversely afiect the life of the tool.

Reciprocation of the piston is generally accomplished in the mannerdescribed in Langas et al. US. Pat. No. 3,106,136, the disclosure ofwhich is incorporated herein by reference. Briefly, fluid under pressureis maintained in the storage chamber 18 as well as in an annular chamber50 between the cylinder 26 and the inner surface of the opening 22 inthe head portion of the tool. The flow of fluid or compressed airbetween the storage chamber 18 and a chamber 60 (between the interior ofthe cylinder 26 and the upper surface of the piston 31) is controlled bya diaphragm valve assembly 52 that seats against the upper edge 54 ofthe cylinder 26. The diaphragm assembly 52 is clamped between a cap 56and the upper edge of the head portion 16 and serves to block off theflow of compressed air between the annular area or chamber 50 and thechamber 60 defined within the cylinder.

The diaphragm valve assembly 52 (FIG. 3) is biased against the upperedge of the cylinder by a spring 62 and high-pressure air introducedinto a closed area 64 between the diaphragm assembly and the cap 56. Thehigh-pressure air is introduced into the area 64 through a tube 66 thatextends through the annularchamber S0 and has its upper end incommunication with the area 64 through a passage 68. The lower end ofthe tube 66 is in communication with a passage 70 defined in the housingwall and communicating with valve means 72.

The valve means 72 (FIGS. 3 and 6) consists of a ball 74 that isnormally maintained in a valve seat 76 located between passageway 70 andan exhaust port 78. In this position, fluid under pressure is deliveredfrom the storage chamber 18 through an opening 80 in alignment with theball 74. With the ball 74 shown in the normal position in FIGS. 1 and 3,the diaphragm assembly 52 is maintained in sealing engagement with theupper edge 54 of the cylinder or sleeve 26 to prevent the flow ofcompressed air into the upper end of the cylinder.

When the diaphragm assembly is seated on the upper edge of the cylinder26, the chamber is vented to atmosphere through an exhaust valve 90.This valve 90 is more clearly shown in FIG. 3 and includes a resilientvalve seat 92 defined in the upper end of the cap 56 and a movableelement 94 carried by the diaphragm assembly 52. In the lowermostposition of the movable element 94, the area above the piston 31 isvented through openings 96 in the cap. However, movement of thediaphragm assembly upwardly will close the valve 90 to seal the areaabove the piston 31 from the atmosphere. It is also desirable to have adiffusing member 98 to divert the flow of air passing through openings96.

When it is desired to actuate the tool, a ball 74 is moved by a valveplunger 84 from its position shown in FIG. 3 to that shown in FIG. 6 bya trigger mechanism 86, which will be described later. Movement of theball upwardly will block the flow of compressed air to the passage andwill simultaneously place the passage 70 in communication with exhaustport 78. This will exhaust the air from the area 64 and the compressedair in the annular chamber 50 will move the diaphragm assembly 52upwardly to close valve and seal the area above the driver assembly 30from the atmosphere. The compressed air on the upper surface of thepiston 31 will drive the piston in the fastener driving direction. Whenthe piston has moved to its downward position, the ball is again allowedto seat on the valve seat 76 and apply fluid under pressure to the area64 and again seal the diaphragm assembly onto the upper edge of thesleeve while venting the chamber 60 through valve 90.

Upon reseating of the diaphragm assembly 52, the piston is returned toits initial position by compressed air entering from annular chamber 50through openings 100 into the cylinder or sleeve wall between pistonflanges 102 and 104 (FIG.6) and the differential area of the pistonflanges will move the driver assembly upward, as is explained in moredetail in the abovementioned Langas US. Pat. No. 3,106,136.

According to the primary aspect of the present invention, the fastenerdriving tool 10 also includes a movable contactor mechanism. Thecontactor mechanism pushes a workpiece toward a second member to whichit is to be secured by a fastener to hold the members to each other orto tighten their connection, or may be proportioned to dimple thework-piece surface. The dimpler mechanism described in detail hereinproduces a dimple or recess around a nail head as a nail is being drivento its final position in wallboard. The dimpler mechanism isreciprocatably driven by the piston 31. When the piston strikes thedimpler mechanism, the piston decelerates while simultaneouslyaccelerating the dimpler mechanism. After that initial acceleration,during which an interposed resilient member is compressed, a metallicpiston portion and a metallic dimpler mechanism portion rigidly engageeach other. Thereafter the piston and metal dimpler portion movetogether to complete the dimpling operation. The dual phase impactdesign results in initial resilient impact between the dimpler mechanismand the piston, and a positive driver-dimpler relationship at the end ofthe piston stroke.

As is clearly shown in FIG. 3, the dimpler mechanism is generallydesignated by the reference numeral 120 and has an inner or upper endportion 122 located within the bore 28 and disposed in the path ofmovement of the driver assembly 30. The dimpler mechanism furtherincludes a second lower or outer end portion 124 below the cylinder,bore 28 that has a dimple forming member 126 at the free end thereofwhich defines a fastener outlet 128 (FIG. 6) through which fastenerspass and which is aligned with the path of move ment of the elongatedriver element 32. The inner and outer portions 122 and 124 areinterconnected through a sleeve 123 on the inner portion 122 and a stud125 on the outer portion that are secured by a pin 129.

The driving means between the driving piston 31 and the dimplermechanism 120 consists of first resilient means interposed between thepiston and the inner end of the dimpler mechanism and carried by one ofthem initially for accelerating the dimpler mechanism while deceleratingthe piston, and rigid striker means on the piston and dimpler mechanismfor direct striking engagement with each other after initialacceleration of the dimpler mechanism and deceleration of said pistonand after the piston and dimpler mechanism are traveling at asubstantially reduced velocity differential. The driving means is mostclearly shown in FIGS. 3 and 6 and includes a resilient member 130 thatis supported on the upper surface of an enlarged flange 132 at the upperor inner end of the dimpler mechanism 120. The resilient member, whichis preferably formed of rubber or other material having elastic memorycharacteristics, defines an opening 134 through which the driver element32 extends. The rigid striker means for direct engagement consists of abore centered stud member or stud 136 that is integral with, and extendsupwardly toward the piston above the upper surface of the enlargedflange 132 and which is located in an opening 138 in the resilientmember. The stud 136 has an enlarged head portion 140 that is receivedin an enlarged portion of the opening 138 and that engages an annularflange 142 defined in the opening 138 intermediate the opposite endsthereof. With this arrangement, the upper surface of the resilientmember 130 is located above the upper surface of the stud 140 and isalso located in the path of movement of the piston.

The rigid means for producing direct engagement between the piston anddimpler mechanism further includes a projection 144 depending from thelower surface of the piston 31 and which is aligned with the stud 136.The projection consists of a bolt which has an enlarged circular head144aand which extends through an opening in the piston. The circularhead or projection 144a is held in fixed relation on the piston by a nut146 threaded on the other end of the bolt.

It should be noted that the longitudinal axes of the projection 144 andthe stud 136 are coincident with each other and are also coaxial withthe axes of the bore 28 and of the piston 31.

Thus, initial engagement between the piston and the dimpler mechanism isthrough the resilient member 130 and the driving piston initiallycompresses the resilient member or bumper resulting in a gradualdeceleration of the piston and a simultaneous acceleration of thedimpler mechanism. When the resilient member has deflected sufficientlythe piston projection 144 directly strikingly engages the confrontingupper surface of the stud 136 to cause simultaneous movement of thedimpler with the piston.

It is desirable that the resilient member has compressibilitycharacteristics such that it will deflect just sufiiciently for thepiston projection to just touch the stud 136 at the terminal portion ofmovement of the piston for a fastener and lumber combination which it isanticipated will require the greatest driving force so that the pistonmoves the dimpler mechanism just enough to produce the desired dimple.With this arrangement, it will be assured that the same dimple will bemade for all grades of lumber and regardless of other factors whichaffect the driving resistance for the fastener. If the drivingresistance is lower, the excess energy transmitted to the dimplermechanism may be dissipated in a manner to be described hereinafter.

It should be noted that the compression of the resilient member willaccelerate the dimpler mechanism and will result in substantialreduction in the velocity differential between the dimpler mechanism andpiston before rigid metal-to-metal contact takes place between them. Itis desirable that the velocity of the piston be reduced so thatapproximately one-half of the total piston energy is used for drivingthe fastener while the remaining half of the energy is used for thedimpling operation. With this arrangement, it is possible to blank firethe tool continuously at normal operating pressure without exceeding theendurance stress limit of the metallic dimpler mechanism and pistonprojection.

According to another aspect of the present invention, the contactormechanism which in this case is a dimpler mechanism incorporates meansfor engaging a work surface and for dissipating excess energy that isapplied to the dimpler mechanism after a dimple has been produced in awork surface. To that end, outer end portion 124 includes a dimpleforming member 126. Member 126 includes a dimpling surface 150surrounding the fastener outlet 128 and an integral expansive energyabsorbing surface segment 152 adjacent the dimpling surface. The surfacesegment 152 has been illustrated as being a collar surrounding dimplingsurface 150, whereas dimpling surface 150 has been illustrated as aspherical segment. As more clearly shown in FIG. 6, surface segment 152is in the form of an expansive flat annular surface. It is disposed at asmall acute angle with respect to the confronting workpiece surface andthe outer periphery thereof merges with an arcuate segment 154 that hasa radius substantially less than the radius of the collar and has itsinner edge merging tangentially with the flat surface while the oppositeedge is directed upward toward the driver assembly. This arrangementmakes it possible to obtain the advantages of the tool even if it isslightly tilted (not exactly perpendicular) in use. Desirably, theforward end of the collar is cut off (shown at 153) to accommodatelocation of said dimpler in close proximity to a comer of a workpiece.

It is preferable that the acute angle defined between the flat surfacesegment 152 and the wallboard surface be substantially less than 30,i.e., less than 30 to a plane normal to the axis of the bore 28, andpreferably about 5 to 7, and that the total area of the flat surface besubstantially greater than the area of the dimpling surface 150.

With this arrangement, and with the driving mechanism for the dimplermechanism that has been described above, a uniform dimpling depth isachieved even though the operating pressure, the lumber grade or thedriving resistance of the fasteners may change, within normallyanticipated ranges. Any excess energy that is developed, such as thatwhich may result from reduced driving resistance, is applied to thesurface segment 152 and will be absorbed and dissipated by the wallboardsurrounding the dimple or recess formed by the dimpler surface. Sincethe total surface area of the surface segment 152 is substantiallygreater than the total exposed surface area of the dimpling surface 150,all excess energy developed will be absorbed by the wallboard with anegligible increase in the dimple depth and without tearing thesheathing or face paper and without crushing the gypsum core.

The dimpler mechanism is positively guided for reciprocable movement onthe housing to insure that it travels along a predetermined path whileit is driven from its retracted to its extended position. For thispurpose, the nose piece 24 mounts a pair of transversely spaceddepending members 158 (FIGS. 4 and 7), each having an elongated slot 160therein. The main body portion of the outer portion 124 is guided formovement between the adjacent surfaces of the members 158 and hasoutwardly directed flanges 162 that are located in the slots 160 (FIG.7).

It should be noted that while the dimpler mechanism has beenspecifically disclosed and described in connection with an eccentricdriver element, the advantages of the dimpler disclosed herein couldreadily be gained by having the outer portion 124 and its fasteneroutlet 128 coaxially aligned with the piston and bore axes.

The fasteners, such as those of a nail strip, to be driven are fed tothe opening above fastener outlet 128 by any suitable means, and amagazine 164 has been illustrated which has an open end in communicationwith a slot 166 that extends from one side of the dimpler (FIG. 7). Themagazine is held in fixed relation on the dimpler mechanism, as by abracket 168.

The tool is also easily accessible for clearing a jam in the dimplermechanism. Heretofore, it has been common to either have a split memberthat defines the fastener receiving opening that could be opened to oneside for clearing a jam or alternatively the opening was slotted theentire length thereof to one side so that access could be gained toremove bent or slightly misformed fasteners. Because of the highshearing stresses that are developed during the use of the high speedfastener tool designed in accordance with this invention, the split nosepiece cannot be utilized and still have a dimpling mechanism that iscapable of withstanding continued use without destruction of parts or itwould be very heavy and bulky. Also, the slotting of the opening theentire length thereof is not feasible because it would eliminate thepossibility of having a uniform dimple surrounding the fastener head.Accordingly, a reduced opening 01' slot 169 is defined in the outer endportion 124 of the dimpler mechanism to provide ready access to anyfasteners which may become jammed in the fastener outlet 128.

As was explained above, the high-speed operation of the tool makes thetool a gun and the fastener being driven by the driver assembly could beconsidered to be equivalent to a bullet when the tool is not placedagainst a workpiece. Thus, the present tool incorporates a safetymechanism that cooperates with the control means for controlling themovement of the piston that prevents firing of the tool when the tool isnot in its operative position with respect to a workpiece. The safetymechanism is generally constructed in accordance with the teachings ofLangas US. Pat. No. 3,194,324, which is incorporated herein byreference. In the present tool, all of the movable elements of thesafety mechanism are substantially enclosed within the tool housing.

a As more clearly shown in FIG. 3, the safety mechanism includes asafety pin that is guided for movement in spaced openings 172 defined inthe head portion 16' of the tool. The pin 170 is normally biased to itsextended position by a spring 174 having one end engaging an annularshoulder on the pin and an oppo site end engaging a surface on the headportion 16. The upper end of the pin cooperates with a trigger lever 176that forms part of the trigger mechanism 86. The trigger lever 176 ispivoted at one end to a trigger member 180 that in turn is pivoted on apin 182 which extends through aligned openings in flanges extending fromthe housing (only one being shown). The opposite end of the lever 176 ispositioned in the path of movement of the safety pin 170 while anintermediate portion of the lever 176 cooperates with plunger 84 thatmoves the valve 74 between its position.

With the safety pin 170 in the position shown in FIG. 3, the triggermember 180 can be moved between the solid and dotted line position showntherein without moving theplunger 84 sufficiently to actuate the valve74. However, if the safety pin is moved to a retracted position shown inFIG. 6, the upper end of the safety pin will carry the free end of thetrigger lever 176 up wardly so that movement of the trigger member 180to its solid line position shown in FIG. 6 will pivot the lever aboutthe upper end of safety pin 170 to actuate the valve. The movement ofthe safety pin 170 between its extended and retracted positions,respectively shown in FIGS. 3 and 6, is accomplished by engagement ofthe dimpler mechanism with a workpiece supporting surface and bymovement of the dimpler mechanism thereagainst to a retracted position.The safety mechanism includes a collar 184 secured to the lower end ofthe pin and having an ann 186 extending therefrom and overlying theupper surface of a projection 187 extending from the enlarged flange 132of the dimpler mechanism 120. Flange 132 is slightly eccentric to permitair to escape while the piston is descending. Projection 187 extendsacross the open area to provide for engagement with arm 186. Thus, ifthe dimpler is moved from the solid to the dotted line position shown inFIG. 3, the flange 132 will carry the collar, and the pin upwardly tomove the safety pin 170 to its release position. This arrangementinsures that the dimpler mechanism is in a proper retracted positionwith respect to the head portion of the tool before the valve means 72can be operated.

The present arrangement also has the desired advantage of being capableof continuously operating the fastener tool without having to repeatedlypull the trigger. Thus, if the trigger member 180 is held in the onposition, the driver assembly 30 will be moved through its power strokeeach time the dimpler mechanism is moved to its retracted position,thereby speeding the driving of fasteners.

As was explained above, the present tool is designed such that duringnormal operation, the tool cannot be actuated without having the dimplermechanism in a retracted position. However, if the tool were to beplaced in position for operation and actuated without proper back-upsupport, the driving and dimpling energy would not be absorbed by theworkpiece and could therefore result in high impact force between thedimpler and the nose piece. According to the present invention, acushioning means or washer 190 is interposed between the housing nosepiece and the adjacent surface of the dimpler mechanism to absorb excessenergy that is developed when the tool is fired without proper oradequate back-up support. Lack of proper back-up support occurs, forexample, when the tool is placed against wallboard at a location spacedfrom a stud or when the wallboard is spaced outwardly from a stud, forexample, at a distance of one-half inch where the wallboard is to besecured to the stud, or when the stud is not properly anchored, hence isloose.

The dimpling tool further includes alignment means carried by the tooland adapted to engage the surface of the workpiece for maintaining theaxis of the bore 28 as well as the axis of the driver element 32substantially perpendicular to the surface of the workpiece. Such ameans is schematically illustrated in FIG. 1 as a U- shaped bracket 194that may be secured to the tool by a common connection that defines theconnection for the magazine. The U-shaped bracket may provide a lowerflat surface 196 that engages the workpiece surface to maintain a properaligned relation of the tool with respect to the surface. Alternatively,an alignment means may be integrally formed with the dimpler mechanism.

The advantages of the present tool have been set forth in the abovedescription that will briefly be summarized at this point. Theconstruction of the tool allows for a fast cycling pneumaticallyoperated fastener driving tool that can precisely limit the terminaldriver position of the dimpler mechanism with respect to the surface ofthe workpiece while preventing the tearing of the thin sheathing ofwallboard around the nail and dimple or recess. The tool also forces thewallboard against the supporting studs while the fastener is beingdriven and allows for concurrent driving of the fastener and productionof the recess or dimple. The eccentric arrangement of the drive elementallows for all fasteners to be driven perpendicularly into a workpieceby the tool and still meet the acceptable building codes relative to thespacing between workpiece comers and the location of the fastener. Thetool produces a substantially constant dimple depth even though thefastener driving resistance varies. The tool is capable of being firedwithout proper back-up support and still have a life time cycle ofmillions. The tool is also easily accessible for clearing a jam and islight in weight with a minimum number of moving parts that can readilybe serviced. Further, it will be realized that hand tools with the typeof action described herein are not limited to drywall or wallboardapplication. They may also be used for fastening thin sheathing, forfastening subflooring or hardwood flooring, or wherever simultaneousfastener driving and tightening to a support or dimpling is desired.

What is claimed is:

l. A fastener driving tool including a housing structure; said housingstructure having a bore therein; a drive piston in said bore; a fastenerdriver cooperating with said piston for driving a fastener; a contactormechanism having an inner end in said housing and which is positioned tobe engaged by said piston and an outer end outside of said housing andwhich is positioned to engage a workpiece; first means interposedbetween said piston and said inner end for accelerating said contactormechanism while decelerating said piston when said piston first engagessaid contactor mechanism; rigid striker means on said piston and saidcontactor mechanism for direct striking engagement with each other aftersaid piston has been decelerated; and said housing structure definingpassages for supplying fluid to one end of said bore for reciprocatingsaid piston.

2. A fastener driving tool in accordance with claim 1, in which saidfirst interposed means comprises a resilient member carried by one ofsaid piston and said contactor mechanism.

3. A fastener driving tool in accordance with claim 2, in which saidinner end defines an enlarged flange, said resilient member beingsupported thereon, and in which said rigid striker means includes a studmember on said contactor mechanism extending upwardly toward saidpiston.

4. A fastener driving tool in accordance with claim 3, and in which saidrigid striker means includes a projection depending from said piston,said projection being aligned with said stud member.

5. A fastener driving tool in accordance with claim 2 includingcushioning means interposed between said contactor mechanism and saidhousing structure for absorbing energy from said contactor mechanism.

6. A fastener driving tool in accordance with claim 1, wherein saidfastener driver is secured to said piston and is laterally offset fromthe central axis of said piston bore, said fastener driver being locatedbetween said central axis and the forward portion of said housingstructure, and counterbalancing means carried by said piston betweensaid central axis and a rear portion of said housing structure.

7. A fastener driving tool in accordance with claim 6, in which saidcontactor mechanism defines a fastener outlet aligned with said fastenerdriver and located below the lower end of said bore.

8. A fastener driving tool in accordance with claim 6, in which saidrigid striker means includes a stud member on said contactor mechanismextending upwardly toward said piston and aligned with said centralaxis.

9. A fastener driving too] in accordance with claim 1 in which saidcontactor mechanism is a dimpler mechanism defining a fastener outletand having a dimpler surface surrounding said fastener outlet, andhaving an expansive energy absorbing surface adjacent said dimplersurface positioned to engage a surface of a workpiece after said dimplersurface engages said surface.

10. A fastener driving tool in accordance with claim 9 wherein saidexpansive energy absorbing surface is a collar surface surrounding saiddimpler surface.

11. A fastener driving tool in accordance with claim 10 in which saidcollar surface is flat and is inclined at an angle of less than 30 to aplane normal to the axis of said bore.

12. A fastener driving tool in accordance with claim 1, includingalignment means carried by said tool and positioned to engage a surfaceof said workpiece for maintaining the axis of said bore substantiallyperpendicular to the surface of said workpiece.

13. A fastener driving tool comprising a housing having a pistonslidably disposed in a bore of the housing and fluid means forreciprocating said piston, a dimpler mechanism slidably supported onsaid housing and having an inner end portion located in said bore, saiddimpler mechanism having a fastener outlet in its outer end, a convexdimpler surface portion surrounding said outlet for engagement with aworkpiece, and an energy absorbing collar portion extending laterallyfrom the periphery of said dimpler surface portion for engaging theworkpiece and for dissipating any energy applied to said dimplermechanism in excess of that required to produce a dimple in saidworkpiece by said dimpler surface portion.

14. A fastener driving tool in accordance with claim 13, wherein saidcollar portion defines an expansive flat surface portion disposed at asmall acute angle to an adjacent surface of said workpiece, said flatsurface having an arcuate peripheral edge.

15. A fastener driving too] in accordance with claim 14, in which saidcollar portion is cut-off to accommodate location of said dimplermechanism in close proximity to a comer of a workpiece, and in which thetotal area of said expansive flat surface is greater than the totalexposed area of said dimpler surface portion.

16. A fastener driving tool in accordance with claim 13, in which saidcollar portion extends completely around said dimpler surface portion.

17. A pneumatically operated fastener driving tool including a housinghaving a circular bore, a circular piston reciprocable axially in saidbore, means for providing a driving force to reciprocate said piston, anelongate driver element secured to said piston, said driver elementbeing offset to one side of the axes of said bore and said piston, andcounterbalancing means secured to said piston on an opposite side of theaxis of said piston for counterbalancing the weight of said driverelement.

18. A pneumatically operated tool in accordance with claim 17, includinga dimpler mechanism having a first portion located in the bore in thepath of reciprocation of said piston, said dimpler mechanism including asecond portion externally of the bore and defining a fastener outletco-axially aligned with said driver element.

19. A pneumatically operated tool in accordance with claim 18, in whichsaid first portion includes an expansive flange and a stud extendingabove an upper surface of said flange, said stud being aligned with theaxis of said bore, and resilient member having an upper surface spacedabove the upper surface of said stud and defining an opening alignedwith said stud.

20. A pneumatically operated tool in accordance with claim 19, furtherincluding a resilient cushion between said flange and said housingadjacent the lower end of said bore for absorbing energy from saiddimpler mechanism.

21. A fastener driving tool including a housing structure, said housingstructure having a bore therein, a piston in said bore, means forproviding a driving force to reciprocate said piston, an elongatefastener driver reciprocable with said piston, a reciprocable contactormechanism postioned to be engaged by said piston for engagement with aworkpiece surface, and control means for controlling the movement ofsaid piston and said contactor mechanism, said control means includingsafety means for preventing operation of said reciprocating means whensaid contactor mechanism is in an outwardly projected position.

22. A pneumatic fastener driving tool having a hous' ing defining a boreand having a driving piston reciprocable in said bore; a dimplermechanism extending outwardly from the lower end of said bore and havingan inner portion located in the path of movement of said piston; saiddimpler mechanism having an outer portion providing an opening forreceiving fasteners; a drive element secured to said piston andreciprocable in said opening in said dimpler mechanism, said innerportion of said dimpler mechanism having a stud extending toward saidpiston; a resilient member surrounding said stud and extending upwardly,said resilient member being located in the path of movement of saidpiston; means for feeding fasteners to said opening for engagement bysaid driver element to be driven into a workpiece through a fasteneroutlet in response to movement of said piston toward said lower end; and

cushioning means between said housing adjacent the lower end of saidbore and said dimpler mechanism for absorbing excess energy developed bysaid piston when said piston engages 'said stud and when inadequateback-up support exists. 7

23. A pneumatic fastener driving tool as defined in claim 22, in whichsaid dimpler mechanism has a spherical segment surrounding said fasteneroutlet and an upwardly and outwardly inclined expansive flat surfacesurrounding said spherical segment.

24. A pneumatic fastening tool having a handle and a head portiondisposed adjacent thereto in which is located a fastener driverassembly; a dimpler mechanism reciprocable on said head portion andadapted to be driven by said driver assembly; means for controlling theoperation of said fastener driver assembly including valve means and atrigger member pivotally connected to said tool, said trigger memberhaving a portion cooperating with said valve means; and a safetymechanism interposed between said trigger member and said dimplermechanism, said safety mechanism including a safety member with biasingmeans cooperating therewith normally to maintain said safety member in afirst position rendering said trigger member inoperative, andcooperating means between said safety member and said dimpler mechanismfor moving said safety member from said first position in response toengagement of said dimpler mechanism with a workpiece and movement ofsaid dimpler mechanism relative to said head portion to render saidtrigger member operative.

25. A pneumatic fastening tool as defined in claim 24, in which saiddimpler mechanism has an enlarged portion adjacent an upper end with areduced stud extending from said enlarged portion toward said driverassembly, a resilient bumper surrounding said stud, seated on saidenlarged portion and extending above a free upper end of said studtoward said driver assembly so that said driver assembly initiallyengages and compresses said resilient bumper and thereafter engages saidstud during a driving stroke.

26. A pneumatic fastening tool as defined in claim 25, in which saiddimpler mechanism has an outer portion having an elongate fastener guideopening therein and said driver assembly includes a driver elementreciprocable in said opening, and in which said outer portion terminatesin a dimpler surface surrounding a fastener outlet, and a forcedissipating surface segment surrounding said dimpler surface.

dimpler mechanism located in the path of movement of the piston, thedimpler mechanism defining a fastener outlet aligned with the driverelement; comprising the steps of: moving said piston and driver elementto engage one end of a fastener and to drive the fastener through saidfastener outlet into the workpiece; decelerating said piston and driverelement during the terminal portion of movement while simultaneouslyaccelerating said dimpler mechanism; thereafter rigidly engaging saiddimpler mechanism with said piston to move said dimpler mechanism,thereby to drive said one end of the fastener below the surface of theworkpiece and to form a dimple in the workpiece surface surrounding thefastener.

28. The method as defined in claim 27, including the further step ofdissipating excess energy developed in said dimpler mechanism withinsaid tool when inadequate back-up support exists.

29. The method as defined in claim 27, including the further step ofdissipating excess energy developed in said dimpler mechanism in thesurface of said workpiece surrounding said recess.

30. A method of operating afastening tool having a piston carrying adriver element and reciprocable between extreme positions with a dimplermechanism located in the path of movement of said piston, comprising thesteps of accelerating said piston and driver element in one direction,decelerating said piston and driver element resiliently whileaccelerating said dimpler mechanism until said piston and dimplermechanism are traveling at a substantially reduced velocitydifferential, and then striking said dimpler mechanism with said pistonduring a terminal portion of movement of said piston, thereby forciblyto push members to be fastened together toward each other, to produce arecess in one member, and to drive a fastener below the surface of saidone member and in the other member.

1. A fastener driving tool including a housing structure; said housingstructure having a bore therein; a drive piston in said bore; a fastenerdriver cooperating with said piston for driving a fastener; a contactormechanism having an inner end in said housing and which is positioned tobe engaged by said piston and an outer end outside of said housing andwhich is positioned to engage a workpiece; first means interposedbetween said piston and said inner end for accelerating said contactormechanism while decelerating said piston when said piston first engagessaid contactor mechanism; rigid striker means on said piston and saidcontactor mechanism for direct striking engagement with each other aftersaid piston has beEn decelerated; and said housing structure definingpassages for supplying fluid to one end of said bore for reciprocatingsaid piston.
 2. A fastener driving tool in accordance with claim 1, inwhich said first interposed means comprises a resilient member carriedby one of said piston and said contactor mechanism.
 3. A fastenerdriving tool in accordance with claim 2, in which said inner end definesan enlarged flange, said resilient member being supported thereon, andin which said rigid striker means includes a stud member on saidcontactor mechanism extending upwardly toward said piston.
 4. A fastenerdriving tool in accordance with claim 3, and in which said rigid strikermeans includes a projection depending from said piston, said projectionbeing aligned with said stud member.
 5. A fastener driving tool inaccordance with claim 2 including cushioning means interposed betweensaid contactor mechanism and said housing structure for absorbing energyfrom said contactor mechanism.
 6. A fastener driving tool in accordancewith claim 1, wherein said fastener driver is secured to said piston andis laterally offset from the central axis of said piston bore, saidfastener driver being located between said central axis and the forwardportion of said housing structure, and counterbalancing means carried bysaid piston between said central axis and a rear portion of said housingstructure.
 7. A fastener driving tool in accordance with claim 6, inwhich said contactor mechanism defines a fastener outlet aligned withsaid fastener driver and located below the lower end of said bore.
 8. Afastener driving tool in accordance with claim 6, in which said rigidstriker means includes a stud member on said contactor mechanismextending upwardly toward said piston and aligned with said centralaxis.
 9. A fastener driving tool in accordance with claim 1 in whichsaid contactor mechanism is a dimpler mechanism defining a fasteneroutlet and having a dimpler surface surrounding said fastener outlet,and having an expansive energy absorbing surface adjacent said dimplersurface positioned to engage a surface of a workpiece after said dimplersurface engages said surface.
 10. A fastener driving tool in accordancewith claim 9 wherein said expansive energy absorbing surface is a collarsurface surrounding said dimpler surface.
 11. A fastener driving tool inaccordance with claim 10 in which said collar surface is flat and isinclined at an angle of less than 30* to a plane normal to the axis ofsaid bore.
 12. A fastener driving tool in accordance with claim 1,including alignment means carried by said tool and positioned to engagea surface of said workpiece for maintaining the axis of said boresubstantially perpendicular to the surface of said workpiece.
 13. Afastener driving tool comprising a housing having a piston slidablydisposed in a bore of the housing and fluid means for reciprocating saidpiston, a dimpler mechanism slidably supported on said housing andhaving an inner end portion located in said bore, said dimpler mechanismhaving a fastener outlet in its outer end, a convex dimpler surfaceportion surrounding said outlet for engagement with a workpiece, and anenergy absorbing collar portion extending laterally from the peripheryof said dimpler surface portion for engaging the workpiece and fordissipating any energy applied to said dimpler mechanism in excess ofthat required to produce a dimple in said workpiece by said dimplersurface portion.
 14. A fastener driving tool in accordance with claim13, wherein said collar portion defines an expansive flat surfaceportion disposed at a small acute angle to an adjacent surface of saidworkpiece, said flat surface having an arcuate peripheral edge.
 15. Afastener driving tool in accordance with claim 14, in which said collarportion is cut-off to accommodate location of said dimpler mechanism inclose proximity to a corner of a workpiece, and in which the total areaof said expansive Flat surface is greater than the total exposed area ofsaid dimpler surface portion.
 16. A fastener driving tool in accordancewith claim 13, in which said collar portion extends completely aroundsaid dimpler surface portion.
 17. A pneumatically operated fastenerdriving tool including a housing having a circular bore, a circularpiston reciprocable axially in said bore, means for providing a drivingforce to reciprocate said piston, an elongate driver element secured tosaid piston, said driver element being offset to one side of the axes ofsaid bore and said piston, and counterbalancing means secured to saidpiston on an opposite side of the axis of said piston forcounterbalancing the weight of said driver element.
 18. A pneumaticallyoperated tool in accordance with claim 17, including a dimpler mechanismhaving a first portion located in the bore in the path of reciprocationof said piston, said dimpler mechanism including a second portionexternally of the bore and defining a fastener outlet co-axially alignedwith said driver element.
 19. A pneumatically operated tool inaccordance with claim 18, in which said first portion includes anexpansive flange and a stud extending above an upper surface of saidflange, said stud being aligned with the axis of said bore, andresilient member having an upper surface spaced above the upper surfaceof said stud and defining an opening aligned with said stud.
 20. Apneumatically operated tool in accordance with claim 19, furtherincluding a resilient cushion between said flange and said housingadjacent the lower end of said bore for absorbing energy from saiddimpler mechanism.
 21. A fastener driving tool including a housingstructure, said housing structure having a bore therein, a piston insaid bore, means for providing a driving force to reciprocate saidpiston, an elongate fastener driver reciprocable with said piston, areciprocable contactor mechanism postioned to be engaged by said pistonfor engagement with a workpiece surface, and control means forcontrolling the movement of said piston and said contactor mechanism,said control means including safety means for preventing operation ofsaid reciprocating means when said contactor mechanism is in anoutwardly projected position.
 22. A pneumatic fastener driving toolhaving a housing defining a bore and having a driving pistonreciprocable in said bore; a dimpler mechanism extending outwardly fromthe lower end of said bore and having an inner portion located in thepath of movement of said piston; said dimpler mechanism having an outerportion providing an opening for receiving fasteners; a drive elementsecured to said piston and reciprocable in said opening in said dimplermechanism, said inner portion of said dimpler mechanism having a studextending toward said piston; a resilient member surrounding said studand extending upwardly, said resilient member being located in the pathof movement of said piston; means for feeding fasteners to said openingfor engagement by said driver element to be driven into a workpiecethrough a fastener outlet in response to movement of said piston towardsaid lower end; and cushioning means between said housing adjacent thelower end of said bore and said dimpler mechanism for absorbing excessenergy developed by said piston when said piston engages said stud andwhen inadequate back-up support exists.
 23. A pneumatic fastener drivingtool as defined in claim 22, in which said dimpler mechanism has aspherical segment surrounding said fastener outlet and an upwardly andoutwardly inclined expansive flat surface surrounding said sphericalsegment.
 24. A pneumatic fastening tool having a handle and a headportion disposed adjacent thereto in which is located a fastener driverassembly; a dimpler mechanism reciprocable on said head portion andadapted to be driven by said driver assembly; means for controlling theoperation of said fastener driver assembly including valve means and atrigger member pivotally connected to said tool, said trigger memberhaving a portion cooperating with said valve means; and a safetymechanism interposed between said trigger member and said dimplermechanism, said safety mechanism including a safety member with biasingmeans cooperating therewith normally to maintain said safety member in afirst position rendering said trigger member inoperative, andcooperating means between said safety member and said dimpler mechanismfor moving said safety member from said first position in response toengagement of said dimpler mechanism with a workpiece and movement ofsaid dimpler mechanism relative to said head portion to render saidtrigger member operative.
 25. A pneumatic fastening tool as defined inclaim 24, in which said dimpler mechanism has an enlarged portionadjacent an upper end with a reduced stud extending from said enlargedportion toward said driver assembly, a resilient bumper surrounding saidstud, seated on said enlarged portion and extending above a free upperend of said stud toward said driver assembly so that said driverassembly initially engages and compresses said resilient bumper andthereafter engages said stud during a driving stroke.
 26. A pneumaticfastening tool as defined in claim 25, in which said dimpler mechanismhas an outer portion having an elongate fastener guide opening thereinand said driver assembly includes a driver element reciprocable in saidopening, and in which said outer portion terminates in a dimpler surfacesurrounding a fastener outlet, and a force dissipating surface segmentsurrounding said dimpler surface.
 27. A method of driving a fastenerinto a workpiece and forming a dimple in the surface of the workpiecearound said fastener with a tool having a reciprocable piston, a driverelement secured to said piston and a dimpler mechanism located in thepath of movement of the piston, the dimpler mechanism defining afastener outlet aligned with the driver element; comprising the stepsof: moving said piston and driver element to engage one end of afastener and to drive the fastener through said fastener outlet into theworkpiece; decelerating said piston and driver element during theterminal portion of movement while simultaneously accelerating saiddimpler mechanism; thereafter rigidly engaging said dimpler mechanismwith said piston to move said dimpler mechanism, thereby to drive saidone end of the fastener below the surface of the workpiece and to form adimple in the workpiece surface surrounding the fastener.
 28. The methodas defined in claim 27, including the further step of dissipating excessenergy developed in said dimpler mechanism within said tool wheninadequate back-up support exists.
 29. The method as defined in claim27, including the further step of dissipating excess energy developed insaid dimpler mechanism in the surface of said workpiece surrounding saidrecess.
 30. A method of operating a fastening tool having a pistoncarrying a driver element and reciprocable between extreme positionswith a dimpler mechanism located in the path of movement of said piston,comprising the steps of accelerating said piston and driver element inone direction, decelerating said piston and driver element resilientlywhile accelerating said dimpler mechanism until said piston and dimplermechanism are traveling at a substantially reduced velocitydifferential, and then striking said dimpler mechanism with said pistonduring a terminal portion of movement of said piston, thereby forciblyto push members to be fastened together toward each other, to produce arecess in one member, and to drive a fastener below the surface of saidone member and in the other member.